1. Field of the Invention
The invention relates to the separation of iso-butane and normal butane. More particularly, it relates to an improved adsorption process for achieving such separation.
2. Description of the Prior Art
Various techniques are known in the art for the separation of normal hydrocarbons from non-normal hydrocarbons. One such separation of practical commercial significance is the separation of iso-butane from normal butane, with said n-butane being used for gasoline vapor pressure blending purposes, and the iso-butane being used for the production of high octane alkylates for gasoline. Other uses of isobutane include the production of tertiary butyl alcohol (TBA), an octane enhancer, and other useful petrochemicals. Iso-butane and normal butane are commonly separated by distillation techniques that are effective but, nevertheless, are highly energy intensive. Adsorption techniques are also known for the separation of branched chain hydrocarbons from straight chain hydrocarbons and can be employed for the separation of iso-butane from normal butane. It will be appreciated that, unlike distillation, adsorption does not require any reflux to produce high purity products. Iso-butane separation from normal butane was the subject of Example 4 of the Kiyonaga patent that disclosed a basic processing feature upon which desirable pressure swing adsorption (PSA) operations in multi-bed adsorption systems have been developed. In the PSA process, a feed gas mixture to be separated is introduced at an upper adsorption pressure to the feed end of an adsorption bed capable of selectively adsorbing a more readily adsorbable component from a less readily adsorbable component of the feed gas mixture that passes through the bed and is removed from the discharge end thereof. The bed is thereafter depressurized to a lower adsorption pressure for removal of the more readily adsorbable component therefrom. In the early development of the PSA process, the adsorption step was carried out until the adsorption front of more readily adsorbable component formed in the bed reached the discharge end of the bed so as to take advantage of the full adsorptive capacity of the bed. Kiyonaga, on the other hand, disclosed the termination of the adsorption step before the adsorption front reaches said discharge end of the bed and before the more readily adsorbable component breaks through into the stream of less readily adsorbable component gas. The adsorption step is followed by a cocurrent depressurization step in which the bed is partially depressurized by the discharge of void space gas, essentially less readily adsorbable component, from the discharge end of the bed. As is disclosed by the Wagner patent, U.S. Pat. No. 3,430,418 and the Fuderer et al, patent, U.S. Pat. No. 3,486,849, the void space gas released during one or more cocurrent depressurization steps is commonly used for pressure equalization purposes between beds and to provide purge gas to a bed in the system at its lower desorption pressure.
In the two bed separation of iso/normal butane as recited by Kiyonaga, a lower desorption pressure in the vacuum range was employed, and a quantity of the less readily adsorbable component was used to countercurrently repressurize the evacuated bed to the upper adsorption pressure. While the PSA process of Kiyonaga, and the cocurrent depressurization feature thereof, constitute a significant advance in the development of the PSA art, there remains a desire in the art for the development of further improvements, as with respect to the separation of iso-butane and normal butane. With respect to this separation, it is desired to employ a process that is less costly to operate than that of Kiyonaga and that employs a less costly adsorption system for the carrying out of the process than is possible in accordance with the teachings of Kiyonaga.
It is an object of the invention, therefore, to provide an improved process for the separation of iso-butane and normal butane.
It is another object of the invention to provide a PSA process capable of enhanced operation in separating said iso-butane and normal butane.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.